1. Field of the Invention
The invention relates to control of the torque being applied to a body by an impact or impulse tool. More specifically, the invention is a method and apparatus that determines a best mathematical expression for representing individual pulses generated by an impact tool and solving the expression to accurately control the torque being applied to a body.
2. Description of the Related Art
Impact tools (also referred to as impulse or pulse tools) are commonly used in the assembly of large fasteners, such as automotive wheel lug nuts. They are able to deliver large torque forces from a physically compact device and can be operated manually.
As described in U.S. Pat. No. 6,655,471 to Cripe, the disclosure of which is incorporated herein by reference, Impax tools operate by applying sequential pulses of torque to a body, in this case a threaded fastener. If the amplitude of the applied torque is high enough to overcome the static friction force of the fastener, the fastener will turn. If the duration of the pulse is short enough, the average operator can manually operate the device. Unfortunately, there is little correlation between the torque within the fastener body applied by an impact tool and the torque observed by the operator. Therefore, impact tools have not been used where accurate control of a fastener torque is important. Rather, controlled-torque assembly processes have been performed manually by an operator with a torque wrench, or in an automated system with a torque-monitored, (non-impact) motor-driven tool, to fine-tune the torque to a pre-determined value. However, these tools are not practical for assembly of large, high-torque fasteners, such as automotive wheel lug nuts.
The most common method of providing the user with a sense of the torque being applied to a fastener is to equip an impulse tool with a torque meter on the tool output shaft. The torque meter is able to electromechanically observe the torque pulses being delivered to the fastener and can be programmed, through an electronic controller, to automatically shut off the impulse tool when a desired torque is reached. A torque meter can produce electronic voltage signals that may be converted to determine the actual torque being applied to a fastener. Some kind of mathematical function is required, however, to convert torque signals from the torque meter into an electronic signal that the controller can use to provide the mechanical feedback (i.e., an automatic shut off) needed by an operator.
Several methods of performing the torque meter data collection and mathematical signal conversion are disclosed in the related art. In U.S. Pat. No. 6,655,471, Cripe et al. disclose using characteristics of a series of pulses to estimate the actual torque being applied to a fastener. In particular, the patent discloses using a collarless torque transducer sensor and induction coils arranged on and around an impact tool shaft to collect imputed torque signals representing the amplitude and duration of each torque pulse. The amplitude and duration of the torque pulse are subtracted from a torque signal and the resulting difference is integrated over time to obtain a fastener angular velocity signal. The angular velocity signal is integrated over time to obtain a displacement signal which can be converted to a torque signal. The resulting estimated torque value is used to determine whether or not to shut off the impact tool.
U.S. Pat. No. 6,311,786 to Giardino et al. discloses using a collarless torque meter and induction coils to collect imputed torque signals acting over a time duration (i.e., an impulse, which is defined as a series of pulses). Knowing the impulse, the torque arm, and the pulse time duration, an accurate measure of the torque can be derived from a determination of the impulse. The impulse value can also be multiplied by a coefficient of proportionality prior to determination of the torque. The coefficient of proportionality is a predetermined value based on the size of the particular tool, e.g., it may vary based on area of magnetic field and manufacturing tolerance. A disadvantage of this method is that it ignores individual pulses and integrates impulses over time.
U.S. Pat. Nos. 5,366,026 and 5,715,894 to Maruyama et al., disclose controlled impact tools in which direct torque measurements are used. Direct torque measurements are made by measuring the force component of torsional stress on a shaft, as exhibited by a magnetic field about a tool output shaft, at the point in time of impact. Torque is related to the force component times the length of torque arm for a particular pulse. One problem with the methods disclosed in those two patents is that the devices measure torque at a given point in time, which may not accurately represent the true torque because torque measurements fluctuate over time, even after a large number of impacts are applied.
Thus, before the present invention, there was no system or method for dynamically calculating the torque being applied to a fastener by an impact tool using the characteristics of individual pulses over a period of time. There remains, therefore, the need for such a system to better control impact tools and prevent under- or over-tightening and loosening of fasteners or other shafts by impact tools.